1. Field of the Invention
The present invention relates to a method for determining coefficients of an equalizer and a device for determining the same.
2. Discussion of the Related Art
Asymmetric high speed digital subscriber line (ADSL) and very high speed digital subscriber line (VDSL) are examples of modern communication systems that permit transmission of data over communication lines at very high rates (e.g., up to 52 Mbits/s). The transmission of high-speed data over band-limited channels may be accomplished by means of discrete multitone (DMT)-based digital communication systems. DMT modems are multi-carrier transmission systems for dividing transmission data into several interleaved bit streams and using these bit streams to modulate several carriers.
Significant limitations in high data rate communication systems are inter-symbol interference (ISI) and inter-channel interference (ICI). One way to compensate for ISI in a DMT system is to add a cyclic prefix to the beginning of each transmitted DMT symbol. Unfortunately, while increasing the length of prefixes reduces ISI, it also decreases the effective data rate. Another approach to combat ISI is to employ an equalizer at the receiver. However, many equalizers need considerable and ongoing computational “overhead”.
In practical communications, the frequency response of a communications channel is not known. Accordingly, equalizers are designed using numerous parameters that need to be adjusted on the basis of measurements of characteristics having an influence on signals of the channel.
A typical equalizer comprises a transversal filter having a delay line spaced by T-seconds, where “T” is the sampling interval and “fs=1/T” is the sampling rate at the receiver. The outputs of filter taps are multiplied by a filter coefficient, summed, and input to a coefficient decision device for selecting coefficients. The coefficient values are typically selected to minimize either peak distortion or mean-squared distortion. The tap coefficients correspond to the channel parameters. Depending on which coefficients are selected, the equalizer can substantially remove the interference from DMT symbols.
There are at least two general approaches to obtain coefficients of an equalizer. One approach is a minimum mean-squared error (MSE) technology to minimize an MSE. Another approach is to obtain an eigenvalue and an eigenvector using the singular value decomposition (SVD). While the SVD approach obtains improved results as compared to the MSE approach, the SVD approach is not widely used in practical communication modems. The SVD approach can be classified into a direct matrix inversion approach or an adaptive algorithm approach. While the adaptive algorithm approach is substantially more efficient than the direct matrix inversion approach, it is not suitable for real-time communications because it is difficult to determine a degree of coefficient convergence. The direct matrix inversion approach is also computationally expensive for the matrix inversion. But since a matrix for inversion is a covariance matrix of a given communication channel, the computational expense is reduced to readily realize the direct matrix inversion approach.
Unfortunately, these approaches are limited in terms of reducing the inter-symbol interference (ISI) and the inter-channel interference (ICI). This is because a receiver installed at the DMT modem knows the frequency response characteristic of a downstream area in a channel for receiving data from a central office, but does not know the frequency response characteristic of an upstream area in a channel for sending data from the receiver to a central office or node along a communications line.
Therefore, a need exists for an equalizer, which amplifies and attenuates a received signal so that the whole band of a channel has a uniform gain, amplifies an upstream area without response and attenuates a downstream area with response. As a result, a channel of a practically used frequency area is attenuated to reduce a signal to noise ratio (SNR).